The paleomagnetic record in carbonaceous chondrites: Natural remanence and magnetic properties

Recent results of an intensive study of the natural remanence (NRM) and the magnetic properties of carbonaceous chondrites (CC) are summarized. It is convincingly demonstrated that the record of ancient magnetic fields has been preserved in these least-altered old samples of solar system material known. Intensities of specific NRM in the 13 meteorites surveyed span a broad range of values from 5 × 10−5 to ∼0.5 emu/g. A low-temperature cleaning technique, based on the memory effect in magnetite grains, was followed by alternating field (af) demagnetization of the residual memory to exhibit the relative stability of NRM in the CC studied. No systematic correlation was found of either intensity or stability of NRM to af demagnetization with petrologic subtype, beyond a trend of increasing stability of memory from C2 to C4. The intensity and stability behavior of the saturation remanence are better suited for use in a magnetic classification of CC. Although type I and II meteorites have very similar stability of memory, they exhibit distinct saturation remanence behavior. In general, CC of types I and II appear more coherent as magnetic groups than type III members. Allende has a uniquely stable natural and saturation remanence. The progressive demagnetization diagrams of NRM showed the presence of a single well-preserved stable paleoremanence component in the meteorites Ivuna (I); Haripura, Cold Bokkeveld, Mighei, Murray, and Renazzo (II); and Vigarano, Coolidge, and Allende (III). The magnetic record in examined samples of Orgueil (I), Mokoia (III-V), and Karoonda (III-O) has been affected by several magnetizing events. The study of viscous remanence acquisition by partially demagnetized CC confirmed that the original NRM has not been significantly changed by residence in earth's field. The unidirectionality and stability of NRM in at least 9 CC indicates that neither strong fields nor high temperatures have modified the original magnetic record. The large coercivites (Hc ≳ 100 Oe) of several CC also reflect the ability to carry stable remanence. Room temperature hysteresis loops and values of magnetic parameters derived therefrom were used to estimate the volume fraction and the relative amounts of single-domain to multidomain magnetic grains. The large anisotropies of magnetic susceptibility found in several CC(P ≲ 2 typically, but up to 5 in Allende) reflect both magnetic and textural alignment of carrier grains.